The indicator turns a different colour depending on the time and temperature of sterilisation:

Strips to be placed on the various parts to be sterilised. Turn from white to black during autoclaving.

Temperature/duration/colour:
121 °C, 3 minutes: light brown
121 °C, 10 minutes: dark brown
134 °C, 0.5 minutes: light brown
134 °C, 2 minutes: dark brown